Curse of the Golden Flower : Why did the emperor poison the consort?

I don't think this is ever properly explained.

As far as I can tell, the emperor is aware of the plot to overthrow him throughout the movie. He is also aware of the illicit affair between his first son and his consort, and has been "for some time". Although his first son attempts suicide rather than go along with the consort's plan to overthrow the emperor. The emperor wonders if the son considered how the emperor would feel. Nevertheless, he blames the consort for seducing his son, and later indicates that his first son truly is/was his favorite son.

So, would you say this is the reason for poisoning the consort?

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Shadows fell on their futile ways, and then there was nothing more

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I took it that he wanted the undivided loyalty and attention of his sons. The empresses' affair with First Son and the fact that her own birth child, Second Son, loved her deeply, perhaps more that he loved his father, tainted them in the Emperor's mind. He could not be sure that their loyalty was totally and unquestionably to him.

Plus it is hinted in the film that it was his political marriage to the Empress (she was a royal princess in her own right) gave him some legitimacy in his grab for the throne, which is why he tried to utterly destroy his first wife and her family. He loved his first wife, and certainly didn't even like his second, even though she helped him to power. She had served her purpose and could be gotten rid of, plus that left him free to find a third empress if he wanted or if it would help him politically.

Extremism is the first choice of the uninformed. Benjamin Whichcote

it's important to keep in mind that prior to the beginning of the film, that the empress had discovered that the emperor's first wife was not dead. because of this, and because the emperor opportunistically 'married into' his position, the discovery that his first wife isn't dead could call into question his legitimacy. it's my suspicion that it was this fact which inspired the empress's attempt at overthrowing her husband. however, the emperor appears to have known what the empress was up to, and conspired to poison her, so that anything she might say about his dead wife would just seem like crazed rambling.